1. Field of the Invention
This invention relates to magnetic bubble domain device structures, in general, and to improved magnetic bubble domain memories using bubble lattice file techniques, in particular.
2. Prior Art
In conventional magnetic bubble domain devices, information is represented by the presence or absence of bubbles and bit positions are defined by device structures such as permalloy T-bar and chevron patterns. However, the storage density of memories using such devices is limited. That is, the domain interactions necessitate a bit separation of at least four bubble diameters, and the minimum dimensions of device structures (which are typically one-half the bubble diameter) place a lower limit on bubble size.
These limitations are relaxed by a new approach called the bubble lattice file (BLF) introduced by Voegeli at the 1974 AIP Conference on Magnetism and Magnetic Materials. In this approach, the bubbles are packed close together in a hexagonal lattice configuration and information is represented by the presence or absence of a pair of Bloch lines within the domain of the bubble. This results in storage densities which, for a given bubble diameter, are up to an order of magnitude greater than for conventional bubble devices. Furthermore, since bit positions are "self-defined," device structures need not be placed at every bit position, thus relaxing lithographic limitations.
A BLF column access system is described by Voegeli et al, in AIP Conference Proceedings, 24, 617 (1974) and Rosier et al, AIP Conference Proceedings, 24, 620 (1974). Basically, the system consists of a storage lattice having n columns of m bubbles, which can be laterally translated, and one or more access channels crossing the storage lattice. Each of these channels contains a shift register for propagating a bubble column and is terminated at opposite ends by read and write stations.
Data is accessed by translating the lattice, either to the left or to the right, until the addressed column is located in the nearest access channel. The bubbles in the addressed column are then propagated along the channel and detected by a suitable detector. The domain pattern remains invariant with translation as buffer columns are inserted and extracted at opposite ends of the storage area, as described in U.S. Pat. No. 3,930,244 by Voegeli.
In known systems, information is represented by the presence (S=0) or absence (S=1) of a pair of Bloch lines within the domain wall of a bubble in a garnet film. The prior art write scheme involves controlling these wall states by a local in-plane field and a critical domain wall velocity. The read function requires discrimination between bubbles with different wall states and sensing by conventional bubble devices. In one approach the discrimination is based on the difference of deflection angles in a field gradient and is inevitably destructive.
Efforts have been made to demonstrate the feasibility of BLF devices. Lattice initialization and translation have been successfully demonstrated using both current access and field access techniques at moderate frequencies. Column translation has been achieved with some degree of success. However, a serious problem encountered is a bias field mismatch between the storage area and the read/write area. Several techniques have been proposed to solve this problem but no satisfactory solution has been reported.
Controllability of the Bloch line wall states has been demonstrated by Hsu, "Control of Bubble Wall States for Bubble Lattice Devices," AIP Conference Proceedings, 24, 624 (1974). However, the stability of these wall states, a key element, in a BLF environment, has not been demonstrated. According to the study performed by Hsu, supra, on isolated bubbles in ion-implanted garnet films, an in-plane field is necessary to stabilize the S=0 state. On the other hand, too large an in-plane field destabilizes the S=1 state. In other words, both the S=0 and S=1 states are "statically" stable in a certain range of in-plane field. Radial and translational bubble motion narrows the stability range. If either the radial wall velocity or the translational bubble velocity reaches the respective critical value, the stability range for the wall states vanishes. The stability range also decreases with increasing temperature. Furthermore, the stability of the wall states may be more sensitive to garnet defects than are other properties such as coercivity. In short, the stability of the wall states is intrinsically much poorer than that of the bubble itself. This casts a serious doubt as to whether this information coding scheme is viable. Moreover, as mentioned earlier, non-destructive read-out does not seem possible with this coding scheme.